Preparation of cyclic siloxanes

ABSTRACT

Hydrolyzable dimethylsilanes containing two hydrolyzable substituents per molecule are contacted with water at a temperature above 200* C. to produce high yields of low-molecular weight cyclic dimethylsiloxanes. The cyclic siloxanes are useful as fluids and as intermediates in preparing silicone rubber polymers.

United States Patent [72] Inventors [54] PREPARATION OF CYCLIC SILOXANES6 Claims, No Drawings [52] US. Cl ..260/448.2 E [5 1] Int. Cl C07f 7/02[50] Field oiSearch 260/4482 E [5 6 References Cited UNITED STATESPATENTS I 3,489,782 1/1970 Pruvost ct al...... 260/4482 E 3,493,5952/1970 Strasser et al 260/4482 E Primary Examiner-James E. PoerAssistant ExaminerWerten F. W. Bellamy Altorneys- Robert F. Fleming, Jr.and Laurence R. Hobey ABSTRACT: l-lydrolyzable dimethylsilanescontaining two hydrolyzable substituents per molecule are contacted withwater at a temperature above 200C. to produce high yields oflow-molecular weight cyclic dimethylsiloxanes. The cyclic siloxanes areuseful as fluids and as intermediates in preparing silicone rubberpolymers.

PREPARATION OF CYCLIC SILOXANES This invention relates to a method forthe preparation of cyclic dimethylsiloxanes.

Cyclic dimethylsiloxanes have become established as commerciallyvaluable materials in view of their utility in the preparation ofpolymeric dimethylsiloxanes. In particular, the compoundhexamethylcyclotrisiloxane' can be polymerized to polydimethylsiloxaneliquids and gums in which the molecular weight distribution of theconstituent molecules falls within a relatively narrow and. However,known methods for the preparation of cyclic dimethylsiloxanes, forexample, the catalytic depolymerization of polydimethylsiloxanes,provide hexamethylcyclotrisiloxane in a relatively low yield and, hence,are not satisfactory for the manufacture of this compound on a largescale.

We have now discovered that when a'dimethylsilane, containing twosilicon-bondedhydrolyzable radicals or atoms, and water are contacted athigh temperatures, cyclic dimethylsiloxanes can be obtained in highyield. We have further discovered that the process lends itself to theproduction of hexamethylcyclotrisiloxane in commercially significantyields. 7

Accordingly, this invention provides a process for the preparation ofcyclic dimethylsiloxanes which comprises contacting at a temperature inexcess of 200 C. (I) an organosilicon compound of the general formula(Cl-l ),SiX,, wherein X represents a substituent which is hydrolyzableunder the reac' tion conditions, and (2) water.

The compound (I) can 2silane containing two siliconbonded methylradicals and two silicon-bonded atoms or radicals which are hydrolyzableunder the conditions of the reaction. Thus, for example, each X canrepresent a hydrogen atom, a halogen atom, preferably chlorine orbromine, an alkoxy radical, an acyloxy radical or an oxime radical.Examples of operative silicon compounds (I), therefore, include (CHgSiBr (CH ),Si(OCH;,),, (Cl-l ),Si(CH Cl. (CH ),Si (OC,H,),,(CH;,),Sil-ICL and (CH Si(O-OC-CH In view of their more generalavailability, the preferred silicon compounds for use in this inventionare those in which X is chlorine, methoxy or ethoxy, the compounddimethyldichlorosilane being most preferred.

The process of this invention involves the reaction in the vapor phasebetween the organosilicon compound and water and can be carried outusing any apparatus which permits the organosilicon compound and waterto be contacted at the desired temperature. Thus, the reaction can becarried out in a horizontal tube or vertical coil provided with suitablefeed and take off lines. Alternatively, the reaction can be carried outin a vertical reactor containinga stirred or fluidized bed of inertparticulate material, for example, ground silica 0r alumina. In such anarrangement, the silicon compounds are introduced at the bottom of thereactor in the region of the bed of particulate material and the cyclicproducts recovered overhead. Any suitable method of maintaining thedesired temperature in the reaction zone can be employed such asinduction heating, electric heating elements or molten salt bath. Othersuitable forms of apparatus for carrying out the process of thisinvention will be readily apparent to those skilled in the art as willvarious modifications and refinements to achieve optimum workingconditions. Thus, the apparatus can be titted with means for removingbyproduced materials, for example, HCl for preventing the carry over ofthe particulate bed with the siloxane product or for recycling unreactedorganosilicon compound.

Contact between the organosilicon compound (1) and the water accordingto this invention should take place at a temperature in excess of 200 C.if significant yields of the cyclic material are to be obtained. Theoptimum temperature for the reaction is in the region of 300 to 400 C.although the actual temperature employed may depend on the particularorganosilicon compound l The process can, if desired, be carried out athigher temperatures, for example, 550 C. and above. However, the use ofvery high temperatures leads to inconveniently short reaction times foroptimum yields of cyclic siloxane product and may also require the useof special reactor materials. The contact time for optimum yield willdepend on the actual temperature employed and may, therefore, varyconsiderably, e.g. from I to 60 seconds. At a temperature of about 400C., a contact time of about 5 to l5 seconds is preferred withcorrespondingly shorter or longer contact times at higher or lowertemperatures.

The relative proportions of the silicon compound and water employed inthe process of this invention are not narrowly critical and can bevaried widely. Preferably at least sufficient water to hydrolyze all ofthe silicon-bonded hydrolyzable radicals is used, from 2 to 8 moles ofwaterper mole of organosilicon compound being the most preferred range.

The products of the process of this invention normally consists of amixture of two or more cyclic methylsiloxanes, the mixture also possiblycontaining unreacted silicon compound and other materials. The desiredcyclic siloxanes can be removed from such a mixture by any suitabletechnique for example, by fractional distillation. When the reactiongives rise to strongly acidic byproducts, these are preferably removedfrom the reaction product at an early stage to minimize loss of thecyclic siloxanes by polymerization. Removal of acidic byproducts can beconveniently achieved by washing the condensed or vaporous reactionproducts with water. Y

The following examples illustrate the invention.

EXAMPLE 1 The apparatus employed in'this example was a mild steel,vertical stirred bed reactor having an internal diameterof 3 inches anda length of 24 inches. The reactor was heated by means of athermostatically controlled, heated air jacket; the temperature of thereactor wall being 400 C. The bed of the reactor consisted of groundsilica having a particle size of approximately l50 microns, the bedhaving a depth of l5 inches. Employing standard metering pumps, waterand an equimolar solution of (CH -,),SiCl in benzene was fed throughseparating lines into the reactor, at its lower end. The (CHhd 3) SiClwas fed into the reactor at a rate of 2 moles per hour (approximately420 g. per hour of solution) and the water at a rate of 8 moles perhour. Prior to entering the reactor, the chlorosilane solution and thewater were vaporized.

The reaction product leaving the reactor was scrubbed with water toremove byproduced hydrochloric acid, condensed and fractionallydistilled. It was found that percent by weight of the (CH;,),SiCl hadbeen converted to cyclic siloxanes. Of the total cyclic siloxane contentof the product, 42 percent by weight was hexamethylcyclotrisiloxane, 34percent by weight was oc'tamethylcyclotetrasiloxane and 17.8 percent byweight was decamethylcyclopentasiloxane.

EXAMPLE 2 The procedure of example 1 was repeated except that thereactor employed was a hollow vertical coil of mild steel, having atotal length of l2 feet and internal diameter of 0.75 inches. About 80percent by weight of the CH SiCl passing through the reactor wasconverted to cyclic siloxanes of which 48.6 percent by weight washexamethylcyclotrisiloxane, 30 percent by weight wasoctamethylcyclotetrasiloxane and 15.6 percent by weight wasdecamethylcyclopentasiloxane.

EXAMPLE 3 The reactor employed in this experiment was a vertical,stirred bed, glass tube having an internal diameter of 1.5 inches and alength of 10 inches. The tube was maintained at a temperature of 310 C.by means of a salt bath. The bed of the reactor was of ground silicasimilar to that employed in example l and the depth of the bed was 7inches.

Into the base of the reactor were introduced, through separate inletsand in the vaporized state, an equimolar solution of (CH Si( OCl-h), inbenzene, at a rate of g. solution per hour, and water at a rate of 2moles per hour. The reaction product was condensed and analyzed, noaqueous wash being carried out on the product. The conversion of silaneto cyclic siloxane was 30 percent and of the cyclic siloxane portion ofthe condensed product 45.9 percent by weight washexamethylcyclotrisiloxane, 45.7 percent by weight wasoctamethylcyclotetrasiloxane and 9 percent by weight wasdecamethylpentasiloxane.

EXAMPLE 4 hydrolyzable under the reaction conditions, and (2) water.

2. A process as claimed in claim I wherein the organosilicon compound isdimethyldichlorosilane.

3. A process as claimed in claim 1 wherein the organosilicon compoundand water are contacted at a temperature within the range from 300 to400 C.

4. A process as claimed in claim 1 wherein from 2 to 8 moles of waterare employed for every mole of the organosilicon compound.

5. A process for preparing dimethylcyclosiloxane polymers comprisingcontacting (l) a sil ane of the general formula (CH SiX, where X is ahydrolyzable substituent, with (2) 2 to 8 moles of water per mol ofsilane l present, at a temperature in the range from 200 to 550 C.

6. The process of claim 5 wherein X is Cl, OCH or OOCCI-l

2. A process as claimed in claim 1 wherein the organosilicon compound isdimethyldichlorosilane.
 3. A process as claimed in claim 1 wherein theorganosilicon compound and water are contacted at a temperature withinthe range from 300* to 400* C.
 4. A process as claimed in claim 1wherein from 2 to 8 moles of water are employed for every mole of theorganosilicon compound.
 5. A process for preparing dimethylcyclosiloxanepolymers comprising contacting (1) a silane of the general formula(CH3)2SiX2 where X is a hydrolyzable substituent, with (2) 2 to 8 molesof water per mol of silane (1) present, at a temperature in the rangefrom 200* to 550* C.
 6. The process of claim 5 wherein X is Cl, OCH3 orOOCCH3.